1. Field of Invention
This invention relates to heating elements, and more particularly to heating elements which are soft, flexible, flat, strong, light and thin, and to their method of manufacture.
2. Description of the Prior Art
Heating elements have extremely wide applications in household items, construction, industrial processes, etc. Their physical characteristics, such as thickness, shape, size, strength, flexibility and other characteristics affect their usability in various applications.
Numerous types of thin and flexible heating elements have been proposed, for example U.S. Pat. No. 4,764,665 to Orbat et. al. This heating element, however, is made of a solid piece of fabric with metallized coating, it does not allow for flexibility in selection of desired power density and is not economical due to metallizing process. The '665 design is also not conducive to hermetic sealing through the heater areas which can cause a short circuit through puncture and admission of liquid. This element can't be used with higher temperatures due to the damage that would be caused to the metallized fabric.
Another prior art example is U.S. Pat. No. 4,538,054 to de la Dorwerth. However, the heating element of de la Dorwerth '054 suffers from the following drawbacks: its manufacturing is complex requiring weaving of metal or carbon fibers into non-conductive fabric in a strictly controlled pattern; the use of the metal wire can result in breakage due to folding and crushing and it affects softness, weight and flexibility of the finished heater; it can't be manufactured in various shapes, only a rectangular shape is available; only perimeter sealing is possible, which can result in a short circuit due to puncture and admission of a liquid; the method of interweaving of wires and fibers doesn't result in a strong heating element, the individual wires can easily shift adversely affecting; the fabric base of the heating element is flammable and may ignite as a result of a short circuit; it is not suitable for high temperature applications due to destruction of the insulating weaving fibers at temperatures exceeding 120.degree. C.
U.S. Pat. No. 3,627,988 describes a method of assembling a surface heater based on carbon fibers consisting of attachment of continuous non-woven carbon fiber material to contact electrodes and to the shape forming layers of fabric by sewing with a thread. The disadvantages of this method are as follows: this method doesn't allow the flexibility of creating heating elements of various shapes and sizes; the manufacturing process is complex and produces hazardous dust during the sewing operation; application of pressure to the surface of the heating element, made of non-woven carbon fabric, significantly increases its electro-conductivity, which, in turn, changes its intended properties; after a period of use under the effect of mechanical forces the non-woven material tends to separate and to form localized lumps affecting usability and performance; this method produces a heater with significant thickness.
Further, attempts have been made to fabricate electrically heated systems from carbon fibers, yarns, and fabrics by coating the carbon material with a protective layer of elastomer or other materials to overcome carbon's extremely poor abrasion and kink resistance (Carbon Fibers for Electrically Heated Systems, by David Mangelsdorf, final report June 1974-May 1975, NTIS). It was found that the coating used in this method reduced the carbon material flexibility and increased the difficulty of making electrical attachments to it, and making electrically continuous seams. The poor flexibility of coated carbon fabric made this material unsuitable for small and complex assemblies, such as handware.
U.S. Pat. No. 4,149,066 to Niibe at. al describes a sheet-like thin flexible heater made with an electro-conductive paint on a sheet of fabric. This method has the following disadvantages: the paint has a cracking potential as a result of sharp folding, crushing or punching; the element is hermetically sealed only around its perimeter, therefore lacking adequate wear and moisture resistance; such an element can't be used with high temperatures due to destruction of the underlying fabric and thermal decomposition of the polymerized binder in the paint; the assembly has 7 layers resulting in loss of flexibility and lack of softness.
Additionally, a known method of achieving a flexible flat heating element is by surfacing threads of fabric with carbon particles and various polymers as disclosed in U.S. Pat. No. 4,983,814. The resulting heating elements have necessary electro-physical characteristics, but their manufacturing is complex and is ecologically unfriendly because of the use of organic solvents, such as diethylphormamide, methylethylketone and others. Furthermore, this method involves application of an electro-conductive layer only to the surface of threads of fabrics. This layer, electro-conductivity of which is achieved through surface contact of extremely small particles, is susceptible to damage due to external factors, such as friction, bending, etc.
The proposed patent seeks to alleviate the drawbacks of the prior art and describes the fabrication of a carbon carrying fabric heating element which is economical to manufacture; doesn't pose environmental hazards; results in a soft, flexible, flat, strong, thin, and light heating element, suitable for even small and complex assemblies, such as handware. A significant advantage of the proposed patent is that it provides for fabrication of heating elements of various shapes and sizes, with predetermined electrical characteristics; allows for a durable heater, resistant to kinks and abrasion, and whose electro-physical properties are unaffected by application of pressure, sharp folding, punches, small punctures, small cuts and crushing.